Background of the Invention
Conventional practices for the removal and recovery of components during drying of coated materials generally utilize drying units or ovens. Collection hoods or ports are utilized in both closed and open drying systems to collect the solvent vapors emitted from the substrate or material. Conventional open vapor collection systems generally utilize air handling systems that are incapable of selectively drawing only the desired gas phase components without drawing the ambient atmosphere. Closed vapor collection systems typically introduce an inert gas circulation system to assist in purging the enclosed volume. In either system, the introduction of ambient air or inert gas dilutes the concentration of the gas phase components. Thus the subsequent separation of vapors from the diluted vapor stream can be difficult and inefficient.
Additionally, the thermodynamics associated with the conventional vapor collection systems often permit the undesirable condensation of the vapor at or near the substrate or material. The condensate can then fall onto the substrate or material and adversely affect either the appearance or functional aspects of the material. In industrial settings, the ambient conditions surrounding the process and processing equipment may include extraneous matter. In large volume drying units, the extraneous matter may be drawn into the collection system by the large volumetric flows of the conventional drying systems.
It would be desirable to collect gas phase components without substantially diluting the gas phase components with ambient air or inert gases. Additionally, it would be an advantage to collect gas phase components at relatively low volumetric flows in an industrial setting in order to prevent the entrainment of extraneous matter.
The present invention provides a method and apparatus for transporting and capturing gas phase components without substantial dilution. The method and apparatus utilize a chamber in close proximity to the surface of a substrate to enable collection of gas phase components near the surface of the substrate.
In the method of the present invention, at least one material is provided that has at least one major surface with an adjacent gas phase. A chamber is then positioned in close proximity to the surface of the material to define a gap between the chamber and the material. The gap is preferably no greater than 3 cm. The adjacent gas phase between the chamber and the surface and the material defines a region possessing an amount of mass. At least a portion of the mass from the adjacent gas phase is transported through the chamber by inducing a flow through the region. The flow of the gas phase is represented by the equation:
M1+M2+M3=M4xe2x80x83xe2x80x83(Equation I)
wherein M1 is the total net time-average mass flow per unit width through the gap into the region and through the chamber resulting from pressure gradients, M2 is the time-average mass flow per unit width from the at least one major surface of the material into said region and through the chamber, M3 is the total net time-average mass flow per unit width through the gap into the region and through the chamber resulting from motion of the material, M4 is the time-average rate of mass transported per unit width through the chamber. For purposes of the invention the dimensions defining the width is the length of the gap in the direction perpendicular to the motion of the material and in the plane of the material.
The present method and apparatus is designed to substantially reduce the amount of dilution gas transported through the chamber. The use of a chamber in close proximity of the surface of the material and small negative pressure gradients enables the substantial reduction of dilution gas, namely M1. The pressure gradient, xcex94p, is defined as the difference between the pressure at the chambers lower periphery, pc, and the pressure outside the chamber, po, wherein xcex94p=pc-po. The value of M1 is generally greater than zero but not greater than 0.25 kg/second/meter. Preferably, M1 is greater than zero but not greater than 0.1 kg/second/meter, and most preferably, greater than zero but not greater than 0.01 kg/second/meter.
In an alternative expression, the average velocity resulting from M1 may be utilized to express the flow of dilution gas phase components entering the chamber. The use of a chamber in close proximity of the surface of the material, and small negative pressure gradients, enables the substantial reduction of the average total net gas phase velocity,  less than v greater than , through the gap. For the present invention, the value of  less than v greater than  is generally greater than zero but not greater than 0.5 meters/second.
The present method attempts to significantly reduce dilution of the gas phase component in the adjacent gas phase by substantially reducing M1 in Equation I. M1 represents the total net gas phase dilution flow into the region caused by a pressure gradient. The dilution of the mass in the adjacent gas phase may adversely affect the efficiency of gas phase collection systems and subsequent separation practices. For the present method, M1 is greater than zero but no greater than 0.25 kg/second/meter. Additionally, due to the relatively small gap between the chamber and the surface of the material, the volumetric flow rate of gas phase components through the gap caused by induced flow is generally no greater than 0.5 meters/second.
The method is well suited for applications requiring the desired collection of vaporous components in an efficient manner. Organic and inorganic solvents are examples of components that are often utilized as carriers to permit the deposition of a desired composition onto a substrate or material. The components are generally removed from the substrate or material by supplying a sufficient amount of energy to permit the vaporization of the solvent. It is desirable, and often necessary for health, safety, and environmental reasons, to recover the vaporous components after they have been removed from the substrate or material. The present invention is capable of collecting and transporting vapor components without introducing a substantial volume of a dilution stream.
In a preferred embodiment, the method of the present invention includes the use of material that contains at least one evaporative component. The chamber is positioned in close proximity to a surface of the material. Energy is then directed at the material to vaporize the at least one evaporative component to form a vapor component. At least a portion of the vapor component is captured in the chamber. The vapor component is generally captured at a high concentration that allows subsequent processing, such as separation, to become more efficient.
The apparatus of the present invention includes a support mechanism for supporting material. The material has at least one major surface with an adjacent gas phase. A chamber is placed in close proximity to a surface of the material to define a gap between the surface and the collection chamber. The adjacent gas phase between the chamber and the material defines a region containing an amount of mass. A mechanism in communication with the chamber induces the transport of at least a portion of the mass in the adjacent gas phase through the region. The transport of mass through the region into the chamber is represented by Equation I. The vapor in the chamber may optionally be conveyed to a separating mechanism for additional processing.
The method and apparatus of the present invention are preferably suited for use in transporting and collecting solvents from a moving web. In operation, the chamber is placed above the continuously moving web to collect vapors at a high concentration. The low volumetric flows and high concentrations of the vapor improve the efficiency of the solvent recovery and substantially eliminate contamination problems associated with conventional component collection devices.
The method and apparatus of the present invention are preferably used in combination with conventional gap drying systems. Gap drying systems generally convey a material through a narrow gap between hot plate and a condensing plate for the evaporation and subsequent condensation of evaporative components in the material. The configuration of the present apparatus, in various locations of a gap drying system, enables further capture of gas phase components which generally can be present in the adjacent gas phase on the surface of the material either prior to entering, or exiting a gap drying unit.
For purposes of the present invention, the following terms used in this application are defined as follows:
xe2x80x9ctime-average mass flowxe2x80x9d is represented by the equation,       MI    =                  1        t            ⁢                        ∫          0          t                ⁢                  m          ⁢                      xe2x80x83                    ⁢          i          ⁢                      xe2x80x83                    ⁢                      ⅆ            t                                ,
wherein M1 is the time-average mass flow in kg/second, t is time in seconds, and mi is the instantaneous mass flow in kg/second;
xe2x80x9cpressure gradientxe2x80x9d means a pressure differential between the chamber and the external environment; and
xe2x80x9cinduced flowxe2x80x9d means a flow generally created by a pressure gradient.
Other features and advantages will be apparent from the following description of the embodiments thereof, and from the claims.